Functionalized Silica Matrices and Palladium: A Versatile

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Functionalized Silica Matrices and Palladium: A Versatile Heterogeneous Catalyst for Suzuki, Heck and Sonogashira Reactions Pitchaimani Veerakumar, Pounraj Thanasekaran, Kuang-Lieh Lu, Shang-Bin Liu, and Seenivasan Rajagopal ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b00921 • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on June 18, 2017

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ACS Sustainable Chemistry & Engineering

Functionalized Silica Matrices and Palladium: A Versatile Heterogeneous Catalyst for Suzuki, Heck and Sonogashira Reactions Pitchaimani Veerakumar,†,⊥* Pounraj Thanasekaran,‡,* Kuang-Lieh Lu,‡ Shang-Bin Liu,†,⊥ and Seenivasan Rajagopal§,* †

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 10617 Taiwan Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan ⊥ Department of Chemistry, National Taiwan University, Taipei 11677, Taiwan § School of Chemistry, Madurai Kamaraj University, Madurai-625021, India ‡

*E-mail: [email protected] (P. Veerakumar); E-mail: [email protected] (P. Thanasekaran); E-mail: [email protected] (S. Rajagopal). ABSTRACT: Remarkable developments have been accomplished in silica-supported palladium nanoparticles (PdNPs) mediated organic transformations for the generation of Suzuki (C-C), Heck (C=C) and Sonogashira (C≡C) coupling reactions in academic as well as industrial communities. Various synthetic methods were adopted to prepare highly dispersed PdNPs encapsulated within various forms of silica supports. The type of reaction examined, size and shape, stability, and recycling ability of silica-supported PdNPs, and the influence of different reaction parameters on carbon−carbon bond forming reactions are discussed. In these reactions, the silica-supported PdNPs exhibited superior performances compared to their unsupported colloidal metal nanoparticle (MNP), revealing the advantages of designing nanocatalysts. Recent progress in the synthesis, catalytic results, stability, and recycling possibilities of silica-supported PdNPs are discussed along with the prospective outlook of relevant research fields. Keywords: Heterogeneous catalysis, Palladium, Silica supports, Suzuki, Heck, and Sonogashira reactions.

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INTRODUCTION Palladium nanoparticles (PdNPs) have become one of the most interesting catalysts because of their size and shape-dependence as well as efficient catalytic activities in catalytic reactions.1−3 Due to high surface energy of palladium, they can easily aggregate to form Pd-black.4 Despite their great contributions, low stability and inconvenient recovery make their use limited.5 PdNPs are preferred as heterogeneous catalysts over homogeneous catalysts because they can be easily separated from the reaction mixture by filtration or centrifugation and reused several times.6 To overcome the degree of aggregation or atom/ion leaching,7 polymers,8 dendrimers,9 ionic liquids,10 surfactants,11 and others12 are employed as stabilizing or capping agents. Thus, organic functionality is a fundamental parameter that could influence their catalytic performance. Encapsulation or immobilization of active nanometals on carbon-based materials,13 metal−organic frameworks,14 alumina,15 polymers,16 zeolites,17 clays,18 and inorganic matrices19,20 have emanated as effective strategies that could enhance their recovery and reduce their tendency to undergo agglomeration.21,22 Usually,

immobilization

is

accomplished

using

covalent

anchoring,23 polymerizations,24 encapsulations,25 sol−gel condensation26 or Stober method.27 For versatile catalytic applications, PdNPs on supports can be protected with organic molecules to enhance their dispersity in solvents.28 Among nanostructured supports, silica supported PdNPs has drawn considerable attention.29 Since the stability of inorganic supports is important under oxidizing conditions, majority of heterogeneous catalysts comes from silica supports. Silica-supported catalysts are advantageous over zeolite-encapsulated catalysts, colloidal nanoparticles and ionic liquid supported metal catalysts (Scheme 1). Amines,30,31 imines,32 phosphine,33 thiols,34 and sulfonic acids,35 could be anchored to the surface of silica, and then, interacted with PdNPs. These materials result in enhancing stability and dispersity in solvents that could allow their practical use in catalytic reactions.

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Scheme 1. Advantages of Using SiO2 as a Support for the Preparation of Heterogeneous MNP-supported Catalysts

Silica matrices including functionalized silica gel,36 silica yolk–shell,37 nanoporous silica,38 SBA-15,39 SBA-16,40 MCM-41,41 MCM-48,42 SiO2 aerogels,43 silica wet gel,44 fluorous silica gel,45 and organosilicas46 have been used to support PdNPs on their surfaces. These materials are utilized in Suzuki-Miyaura, Heck and Sonogashira cross-coupling reactions because they possess excellent reactivities under sustainable conditions and, can be readily recovered and reused by simple filtration after completion of the reaction. This straightforward workup process and simple handling of catalysts is extremely practical applications for industrial-scale catalysts.47 Encouraged by catalytic applications of PdNPs, more details on C-C coupling reactions of PdNPs can be found in several reviews.1,2,16,48−50 However, no detailed coverages for the use of heterogeneous PdNPs immobilized on various forms of SiO2 supports have been reported. Thus, silica-supported PdNPs catalyzed cross-coupling reactions are very important subjects, and reviewing these would be timely. This review covers the recent progress on syntheses, morphological features and catalytic performances of various SiO2 matrices decorated PdNPs with respect to Suzuki, Heck and Sonogashira coupling reactions and their future prospects are discussed.

Solid-supported Palladium Nanocatalysts: Coupling Reactions. Silica-supported PdNPs play a significant role in heterogeneous catalysis because of low cost, accessibility, high stability, and large surface area with excellent porosity. They 3



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provide the advantages of reaction pathways under mild reaction conditions and comprise environmental-friendly processes toward sustainable chemistry.51,52 The C−C coupling reactions including Heck, Suzuki, Negishi, Corriu-Kumada, Sonogashira, Stille, Hiyama, Tsuji-Trost, and Ullmann reactions have highlighted the practical applications of PdNPs in chemical industries. The types of Pd catalysts that have been applied in Suzuki–Miyaura, Heck and Sonogashira reactions and the desired products obtained are most important in the field of pharmaceutical industries. Thus, silica-supported PdNPs catalyzed cross-coupling reactions are well-known and very important subjects, thus addressing their use is a timely topic.

Recent Progress in Suzuki-Miyaura Cross-coupling Reactions. Palladium catalyzed carbon−carbon bond forming reactions developed by Suzuki have made a significant impact in the field of organic chemistry.53 It is a versatile route for the construction of biaryls, which are partial structures in pharmaceuticals,54 by the reaction of aryl halides with PhB(OH)2 using heterogeneous PdNPs under milder condition. Marck and coworkers reported the first example of Pd/C catalyzed Suzuki reaction.55 Reetz and co-workers56 first demonstrated the use of Pd and Pd/Ni NPs for the Suzuki coupling of arylbromides and chlorides with Ph(OH)2 using 2 mol% Pd catalysts (Scheme 2). The Pd/Ni NPs had more activity for the transformation of aryl chlorides.

Scheme 2. First Example of Pd and Pd/Ni NPs Catalyzed Suzuki Coupling

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In 2004, PdNPs (2−5 nm) entrapped in SiO2 gel, (SiO2/TEG/Pd, 1) were prepared by heating a mixture of [Pd(PPh3)4], tetra(ethylene glycol) and tetramethoxysilane via a sol-gel process followed by treatment with water (Scheme 3).57

Scheme 3. Preparation of Recyclable Catalyst (1)

Aryl-iodides and bromides were coupled with Ph(OH)2 using 0.75 mol% Pd catalyst, K3PO4 in toluene at 110 °C to give their products in 86−100% yields, although aryl chlorides were less efficient ( Pd@pSiO2 before thermal treatment (72%) > Pd@SiO2 core-shell NPs (40%) > free-standing PdNPs (35%). The superior catalytic activity of (3) was attributed to the completely exposed surface of Pd cores, rapid diffusion of the reactants through SiO2 layers and clean metal surface aided by high temperature. Under identical condition, bromo- and chloro-benzenes gave their products in 61−100% yields. These catalysts exhibited a high turnover frequency of 78,000 h−1, as well as being recycled 10 times, indicating the heterogenesity of (3). Owing to large surface area, pore volume, as well as adjustable pore size of mesocellular foam,60 Bäckvall group developed Pd0-AmP-MCF (4) catalyst using aminopropyl-functionalized siliceous foam with PdNPs.61 The Suzuki reaction gave a four-fold decrease in yield under conventional heating instead of MW irradiation,62 indicating the unavailability of efficient heat transfer to the PdNPs. This catalyst was active in the coupling of aryl-iodides and bromides (longer reaction times with higher catalyst loading) with Ph(OH)2 under optimized conditions to give their products in 97–99% and 40−99% yields, respectively. This catalyst showed good performance in the coupling of hetero-arylhalides with hetero-arylboronic acids under MW irradiation.63 This catalyst could be recycled three times with a minimal Pd leaching and aggregation. 6


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Using different Pd sources, and silyl-functionalized KIT-6 with different pore sized PdNPs (5−12 nm), KIT-6(X)-SH (5a-d), (X = 60, 80, 100 and 130 °C), was prepared under hydrothermal condition.64 All catalysts underwent the Suzuki-Miyaura reaction between p-bromoacetophenone and pinacol ester of Ph(OH)2 using 1.0 mol% Pd catalyst in DMF/H2O at 80 °C to give 90−100% yields. These catalysts were reused 6−8 times, which are higher than SBA-15-based materials. However, the activity was decreased due to the formation of large PdNPs and Pd leaching. A Pd(II)–SBA-16 (6) catalyst was prepared by the reaction of functionalized SBA-16 with PdCl2 (Scheme 6).65 This catalyst showed a type-IV isotherm with H2 hysteresis loop, indicating the presence of mesoporous cage-like structure.

Scheme 6. Preparation of the Catalyst Pd(II)–SBA-16 (6)

Under optimized conditions (0.5mol% Pd catalyst, K2CO3, EtOH/H2O, 80 °C), aryl-iodides and bromides coupled with arylboronic derivatives to give the desired products in 94−100% and 81−95% yields, respectively, but, aryl chlorides did not couple. This catalyst could be reused five times. Song et al66 prepared a nanoreactor (7) that was composed of hollow spheres of silica with PdNPs located inside the pores. HRTEM images showed the distribution of PdNPs (2−10 nm) on SiO2. This catalyst showed 99.5% activity in the coupling of aryl iodides with Ph(OH)2 using 10 mg Pd catalyst and K2CO3 in refluxing ethanol. However, bulky groups on the Ph(OH)2 failed to couple. Commercial Pd/C catalyst (Pd/C, 8)66 performed well in the Suzuki reaction (90% yield) without shape selectivity. However, the shape selectivity of 1-naphthaleneboronic acid using (7) was obtained by the collective diffusion barriers from reactants. A PNP-SSS catalyst (9) was synthesized by immobilizing palladium nanoparticles (PNP) on a silica-starch substrate (Scheme 7).67 By selecting appropriate boronic acids and di-arylhalides under optimized conditions (0.08 g Pd

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catalyst, water, NaOH), a wide range of p-teraryls could be synthesized in excellent yields. This catalyst could be reused six times without Pd leaching.

Scheme 7. Synthetic Route for Preparation of PNP-SSS Catalyst (9)

The fabrication of SBA-15–Pd-X (X = 1, 2, 5, 10, 20, and 50% Pd, 10a-f) and MSU-2–Pd-X (X = 1, 2, 5, 10, 15 and 20% Pd, 10a-f) by functionalizing SBA-15 or MSU-2 through the reduction of [PdCl2(cod)] was reported.68 TEM images showed that these catalysts were spherical in shape with a diameter of 30 and 28 nm, respectively. Higher conversions were obtained, when these catalysts were tested in the Suzuki reaction of boronic acids with m-bromoanisole or o-bromopyridine using DMF–H2O and K2CO3 at 70 °C. A

SiO2-acac-PdNPs

(11)

was

prepared

by

the

reaction

of

acetylacetone-modified silica with Pd(OAc)2 in the presence of CH3CN (Scheme 8).69 TEM images indicated the formation of PdNPs (6–12 nm).

Scheme 8. Synthetic Route for Preparation of the Catalyst (11)

The Suzuki coupling of aryl- iodides and bromides with Ph(OH)2 was rapid using 0.04 mol% catalyst, NaHCO3, in refluxing H2O to give their products in 8


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91−94% yields. However, arylchlorides reacted slowly to give 84−88% yields. In chemoselectivity reaction, Br is a better leaving group compared to Cl, giving the products in 85−91% yields. This catalyst was stable in air and water and could be reused six times. Starting from the reaction of Schiff-base-functionalized silica with Pd(OAc)2 in MeOH, Mahjoub and co-workers41 prepared MCM(Pd)-41 catalyst (12). HR-TEM images of (12) showed the distribution of PdNPs in pore channels. Excellent yields (80−95%) were obtained when aryl-iodides or bromides reacted with boronicacids under solvent-free condition using 0.01 g Pd catalyst and K2CO3 at 100 °C. This catalyst could be recycled ten times. Sarkar

group70

prepared

a

SBA-15-Pd

catalyst

(13)

by

treating

mercaptopropylated SBA-15 with (CH3CN)2PdCl2 in CHCl3. Using 0.1 mol% Pd-catalyst, the coupling of aryl-iodides and bromides with Ph(OH)2 in the presence of K2CO3 in aqueous ethanol at 90 °C gave their biaryls in 90−97% and 87−94% yields, respectively. However, the coupling of arylchlorides gave 51−54% yields. This catalyst could be reused three times. Using a modified Aerosil-380, SBA-15, plugged SBA-15, and m-MCF with thiol ligands, SiO2-SH•Pd (14), SBA-15SH•Pd (15), plugged SBA-15SH•Pd (16) and m-MCF-SH•Pd (17) catalysts with the same sized PdNPs (2 nm) were prepared.34 Catalyst (16) was active in the Suzuki reaction of Ph(OH)2 with p-iodoanisole to yield 80% conversion under optimized condition (ethanol, Na3PO4●12H2O, 0.12 mol% Pd catalyst, 60 °C), while bromo- and chloro-benzenes in DMF gave their products in 80 and 10% yields, respectively. Based on structural characteristics, catalyst (16) showed the best BET surface area, pore volume and recyclability compared to other supports. A Pd@imine-SiO2 (3.26 nm) catalyst (18) was synthesized by treating imine functionalized SiO2 with Pd(OAc)2 in acetone (Scheme 9).71 Scheme 9. Preparation of the Supported Schiff-base Pd(II) Complex (18)

9



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With optimized conditions (0.463 mol% Pd catalyst, iPrOH/H2O, Na2CO3, RT), catalyst (18) showed 90−100% yields for the Suzuki reactions of 4-arylbromides with Ph(OH)2 derivatives. However, arylchloride did not couple. This catalyst could be reused eight cycles without Pd leaching. Paul and co-workers72 developed Pd(0)-EDA/SC-1-3 (19a), Pd(0)-EDA/SC-2 (19b) and Pd(0)-EDA/SC-3 (19c) based on immobilization of PdNPs on ethylenediamine-functionalized silica-cellulose. SEM and TEM images confirmed uniform distribution of PdNPs on EDA/SC. Among these catalysts, catalyst (19b) was active in the Suzuki reaction of arylbromides with arylboronic acids under optimized conditions (K2CO3, TBAB, 2.5 mol% Pd catalyst, water, MW) to give 75−92% yields. Under identical condition, thermal coupling gave similar % product yield. This catalyst was reused five times without loss of activity. A Pd–SO3H/SiO2 catalyst (20) was prepared by the reaction of Pd(OAc)2 with silica-supported phenylsulfonicacids.35 Catalysts 0.25% wt% Pd catalyst (20) and 0.5%wt% Pd catalyst (20) onto silica were also prepared for comparative studies. TEM images showed that spherical PdNPs with sizes of 10–16 nm and 37–45 nm for 0.25% (20) and 0.5% (20), respectively, were distributed onto silica. The coupling of anilines with boronic acids was performed to give 52−84% yields using 50 mol% H+ and 3.7 mol% Pd. This protocal was applied to the preparation of fungicide Boscalid, which was obtained by coupling of o-nitroaniline with p-Cl-Ph(OH)2. Working with a higher catalyst loading allowed the catalyst to be efficiently recycled. Zhao et al3 prepared Pd-MSNSs-T (T = 10, 30, 50 and 70 °C, 21a-d) through amino-functionalized silica with K2PdCl6 in the presence of NaBH4. TEM images of (21c) showed that PdNPs were 1.3−5 nm in size. Among these catalysts, the immobilized PdNPs in (21b) retained their locations in channels without aggregation. The Suzuki reaction proceeded with 83−96% conversion, when aryl-bromides and iodides were coupled with Ph(OH)2 using 0.075−0.75 mol% Pd catalyst, and K2CO3 in MeOH. This catalyst can be reused six cycles without apparent deactivation. 10


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A phosphine-free Pdnp-nSTDP (22) catalyst was prepared based on PdNPs immobilized on nano-silica polymer (Scheme 10).23 TEM images indicated that dendritic polymer would be a good host for 3.1 nm sized PdNPs.

Scheme 10. Synthesis of the Pdnp-nSTDP Catalyst (22)

Under optimized condition (0.006 mol% Pd catalyst, K2CO3, DMF/H2O), aryl-iodides and bromides were coupled with Ph(OH)2 to give the desired products in 90−96% yields at RT or 91−96% yields under MW. By increasing the temperature and reaction time, the coupling of aryl chlorides gave biaryls in high yields. The efficiency of (22) was examined for the synthesis of star- and banana-shaped molecules and the yields were 20−91% at RT and 30−94% under MW irradiation. This catalyst could be recycled seven times without metal leaching. A nanocatalyst “Nano-Pd/SiO2” (23a) prepared from PdNPs on SiO2 via a chemical vapor deposition technique was reported.73 SEM and AFM images indicated that PdNPs (30 nm) was uniformly distributed on the surface of SiO2. A 5% Pd/SiO2 11



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(23b) was also prepared for comparing the reactivity purpose. Suzuki reaction of Ph(OH)2 with p-iodophenol in H2O at RT using K2CO3 showed that catalyst (23b) was less reactive (85% yield) compared to that (23a, 95% yield) due to aggregation of PdNPs. Under optimized condition (0.3 mol% Pd catalyst, K2CO3, H2O), the desired products were obtained in 82−95% yields at RT from the reaction of p-iodo- and bromo-phenols with Ph(OH)2. Halophenols showed a high selectivity, which was attributed to H-bonding between the OH groups of the silica and halophenols. This catalyst could be recycled four times without Pd leaching. The reaction of PdCl2 with SiO2-diethanolamine-1,4-diaza-bicyclo[2.2.2]octane followed by reduction with hydrazine afforded DEA-DABCO-Pd(0) (24).74 TEM images of (24) showed that the size of the PdNPs was 10−20 nm. The Suzuki reaction of o- and m-nitroaryl halides gave trace amounts of product, while p-arylbromides afforded coupling products in 92−99% yields under optimized condition (50% EtOH, K2CO3). However, para-substituted aryl chlorides were less reactive to give their products in 77−89% yields, when K3PO4, 0.08 mol% Pd catalyst and DMF was used at 130 °C. This catalyst reused five cycles without significant loss of activity. A silica–terpy–Pd(II) nanocatalyst (25) was prepared by the reaction of SiO2– terpy with Pd(OAc)2 at RT.75 This catalyst was active in the Suzuki coupling of aryl iodides with arylboronic acids using 1.0 mol% Pd catalyst and K2CO3 in water at 90 °

C to give 86-91% yields. The coupling of chloro- and bromo-benzenes failed to

couple. This catalyst was reused four times without Pd leaching. A silica tethered Pd–DABCO catalyst, Pd–DABCO@SiO2 (26),76 was prepared by the reaction of DABCO-functionalized silica with Pd(OAc)2. When the optimized protocol (K2CO3, 1.0 mol% Pd catalyst, air, 80 °C) was applied to reactions of aryl bromides with arylboronic acid, the coupling products were formed in 70–96 % yields. For arylchlorides, a much longer time was required, giving 20−76% yields. This protocol

was

extended

to

the

Suzuki

cross-coupling

of

7-bromo-2,3-diphenylpyrido[2,3-b] pyrazine, which are structures found in anticancer agents and antibiotics, with arylboronic acids using K2CO3 in THF under reflux to give the desired products (79−84%). A hot filtration test proved the heterogenesity of (26).

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Organic-functionalized NH-SBA-15 reacted with PdCl2 via the coordination bonds followed by reduction with formalin to obtain Pd-SBA-15 (27a).77 Three more samples were obtained using 0.05 g PdCl2 for (27b), 0.2 g PdCl2 for (27c) and SBA-15 for (27d) to compare the reactivity. TEM images confirmed the encapsulation of PdNPs (3–6 nm) in the silica pores. These catalysts were active in the Suzuki reaction of aryl bromides with Ph(OH)2 to give their biaryls. Among others, catalyst (27d) showed the lowest activity with a high degree of Pd leaching, indicating the importance of amino groups in preventing agglomeration and leaching of PdNPs. The catalyst (27a) showed highest activity with a moderate Pd leaching, as it contained optimal N:Pd molar ratio. Under optimized conditions (K2CO3, 0.05 mol% Pd catalyst, water, 80 °C), Suzuki reaction of bromobenzenes with Ph(OH)2 gave 90−99% yields of biaryls. This catalyst could be reused six times with a minor aggregation and Pd or N leaching. Polymer-encapsulated silica supported Pd(0) catalysts (28a and 28b) were prepared by the reaction of amine-modified silicas with Pd(OAc)2, followed by reduction with NaBH4.78 TEM images showed that PdNPs (100 nm) were surrounded by polymer coating. Under optimized conditions (MeOH:H2O:DME, DIPEA, 200 mg Pd catalyst, 1.5 g sand, 120 °C), these catalysts showed excellent reactivity (>95%) with deactivated aryl iodides and a lower conversion with activated aryl iodides (56−86% for 28a; 19−43% for 28b). Both catalysts were used for >50 hours of consecutive operation with a minimal Pd leaching. Das group32 has developed a Pd@imine–SiO2 (29) by immobilizing Pd(OAc)2 onto silica through Schiff-base condensation between APTES-functionalized silica and acetamide. SEM studies suggested that the presence of Pd caused a decrease in silica size. This catalyst was active in the Suzuki coupling of aryl bromide and iodides with Ph(OH)2 using 0.08mol% Pd catalyst and K2CO3 in iPrOH/H2O at 60 °C to yield their products in 84−99% and, 96−98%, respectively, but the coupling of aryl chlorides was poor. This catalyst can be reused six runs without Pd leaching. Zhang

group

prepared

a

shell-in-shell

structured

@Pd/meso-TiO2/Pd@meso-SiO2 (30a) as shown in Scheme 11.54

13


nanocatalyst,


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Scheme

11.

Schematic

Illustration

Showing

the

Synthesis

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of

the

‘‘Shell-in-Shell’’@Pd/meso-TiO2/Pd@meso-SiO2 Nanocatalyst (30a)

EDX confirmed the presence of PdNPs (∼5 nm) in (30a). Contrasting catalysts, @meso-TiO2/Pd@meso-SiO2

(30b),

@meso-TiO2/Pd@SiO2

(30c),

@Pd/meso-TiO2/Pd (30d), and @meso-TiO2/Pd (30e) were also prepared to compare catalytic performance. When the catalyst (30a) was explored in the Suzuki coupling of iodobenzenes with Ph(OH)2 under optimized conditions (80 °C, EtOH, Cs2CO3, 10 or 25 mg Pd catalyst), it showed a 53−100% conversion and TOF of 8220−15,546 h−1, which were higher than (30b-e). The mesopores in the SiO2 shell allowed the reactants to selectively diffuse into the nanoreactors, thus achieving catalytic selectivity. This catalyst showed outstanding performance in this reaction. Cao and Song37fabricated NC/Pd@mSiO2 nanoreactor (31), as shown in Scheme 12. TEM images showed that 5 nm sized PdNPs were uniformly distributed on the surface of PDA spheres.

Scheme 12. Synthetic Strategy for NC/Pd@mSiO2Yolk–shell Nanoreactor (31)

This nanoreactor was active in the Suzuki coupling of bromo- and iodo-benzenes with Ph(OH)2 in the presence of K2CO3 and 10 mg Pd catalyst in ethanol/H2O at 80 °C. The conversion of bromo- and iodo-benzenes reached 99%, but chlorobenzene gave only 26%. This nanoreactor was reused five times without Pd leaching. A functionalized KCC-1-NH2 composite was reacted with PdCl2 followed by hydrogen reduction to afford KCC-1-NH2/Pd nanocatalyst (32).79 TEM images of (32) 14


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demonstrated the loading of PdNPs (1–5 nm) into fibers of KCC-1. Under optimized conditions (K3PO4, EtOH/H2O, 100 °C, 0.5 mol% Pd catalyst), this catalyst catalyzed the coupling of arylhalides with arylboronic acids to produce the corresponding biaryls in 77−97% yields. The catalyst could be reused seven times without Pd leaching. A nanocatalyst Pd@SiO2 (33) was prepared by heating silylated Pd complex with silica gel in toluene at 100 °C.80 This catalyst showed 80−94% yields in the coupling of heteroaryl chlorides with arylboronic acids using 0.5 mol% Pd catalyst at 60 °C in water. These reactions gave surprising results, since chlorothiophenes in the coupling reaction are challenging due to the strong affinity for Pd.81 TBAB was used as a phase transfer to enhance the reactivity in water. The catalyst could be reused six times with a minor Pd leaching. As silica KIT-6 possesses large tunable pores with thick pore walls, high stability and high surface area with large pore volume, nanocomposite Pd-PHEMA/KIT-6 (34) was fabricated by the reaction of poly(2-hydroxyethyl methacrylate)/KIT-6 with Pd(OAc)2 through an in situ polymerization method.82 Under optimized condition (0.14g Pd catalyst, KOH, water, 40 or 90 °C, air), the coupling of aryl iodides with Ph(OH)2 gave 85−97% yields, while bromo- and chloro-benzenes yielded 90−95% at 40 or 90 °C. This catalyst could be reused nine times with a low level of Pd leaching. Hollow Pd spheres (35) were prepared by heating mercaptopropylsilylfunctionalized silica with Pd(acac)2 at 250 °C followed by treatment with HF.83 TEM images showed that the shell was composed of 10 nm sized PdNPs. Suzuki coupling between Ph(OH)2 and C6H5I was conducted using 3mol% Pd catalyst, and K3PO4 in EtOH at 78 °C to produce the desired bi-aryls in 96−99% yields. However, aryl bromides required 5 times more Pd catalyst to achieve a similar yield, while aryl cholorides did not couple. This catalyst could be reused seven times without Pd leaching. Two effective catalysts, SBA-15-SH●Pd (36a) and SBA-15-NH2●Pd, (36b) were prepared by the reaction of mercaptopropyl- or amine-modified SBA-15 with Pd(OAc)2.84 The catalyst (36a) worked well in the Suzuki coupling of arylchloride with arylboronic acid using 2mol% Pd catalyst and K2CO3 to give product in 96% 15



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yield in H2O. Bromobenzenes gave 82−97% yields in H2O or DMF/H2O. It could be reused four times with a low extent of Pd leaching. Sangtrirutnugul group prepared a T10-Pd catalyst (37) from a reaction between Pd(COD)Cl2 and pyridine–triazole-functionalized decameric silsesquioxane.85 XPS and XRD analyses of (37) confirmed the presence of Pd. Under optimized conditions (1.4mol% Pd catalyst, K2CO3, EtOH:H2O and 60 °C), coupling products derived from bromoarenes and Ph(OH)2 were obtained in 65−98% yields. This catalyst could be recycled five times with no detectable deactivation. Cai group86 developed a MCM-41-2N-Pd(II) catalyst (38) by heating 3-(2-aminoethylamino)propyl-functionalized MCM-41 and Pd(OAc)2. XPS studies confirmed the presence of Pd in this catalyst. This catalyst was active in the coupling of p-arylbromides with Ph(OH)2 under optimized conditions (0.2 mol% Pd catalyst, K2CO3, xylene, 90 °C) to produce their desired products in 91−97% yields, while o- or hetero-arylbromides afforded 83-89% yields. This catalyst could be recycled ten times without Pd leaching. Strawberry-like nanomaterials, NH2-SiO2@Pd (5.1 nm, 4.47wt% Pd catalyst (39a), and 6.0 nm, 5.95wt% Pd catalyst (39b)) and PPh2-SiO2@Pd (39c), were prepared

by

the

reaction

of

2-(diphenylphosphino)ethyltriethoxysilane-functionalized respectively.

87

silica

APTES-

or

with

Pd(II),

TEM images showed that 3.6 nm sized PdNPs were dispersed on the

silica surface. The Suzuki coupling reaction between Ph(OH)2 and aryliodides catalyzed by (39a) and (39b) using 0.1 mol% Pd catalyst and K2CO3 in DMF–H2O at 100 °C produced biphenyls with a 94−96% conversion. A SiO2-pA-Cyan-Cys-Pd (40) catalyst based on propylamine-cyanuric-cysteine Pd complex immobilization on silica was prepared.88 TEM images showed the size-distribution of PdNPs (15−30 nm) on the surface of silica. This catalyst was efficient for the Suzuki reaction of aryl iodides or bromides to give 87−95% yields using 0.5 mol% Pd catalysts, K2CO3, and H2O at 100 °C. However, this catalyst provided 52−61% yields for chloroarenes. This catalyst could be recycled five times without losing its activity. Sarkar et al89 prepared a silica supported polyethyleneglycol-encapsulated Pd catalyst (41) by treating Fischer carbine acyl metal salt with a mixture of K2PdCl4 and 16


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functionalized silica in water (Scheme 13). TEM images of (41) confirmed that the spherical PdNPs (12–14 nm) were dispersed across the silica. It catalyzed the Suzuki coupling of Ph(OH)2 with arylbromides and iodides using K2CO3 and 1 mol% Pd catalyst in DMF at 100 °C to generate their products in 88−95% yields. The catalyst can be reused four cycles with no Pd leaching.

Scheme 13. Synthesis of Silica Supported PdNPs (41)

Jin group90 prepared ionic liquid supported Pd catalyst (42) by immobilization of triethoxysilylated ionic liquid on silica followed by reaction with Pd(OAc)2 (Scheme 14). Under optimized conditions (0.1mol% Pd catalyst, Na2CO3, DMF:H2O, 65 °C), aryl-bromides, and iodides underwent coupling with Ph(OH)2 to give their desired products in 83−100% yields, but 52−66% yields were observed with aryl chlorides. This catalyst could be reused six times without Pd leaching.

Scheme 14. Preparation of the Ionic Liquid supported Pd Catalyst (42)

Organically modified silicas with different concentrations were reacted with Pd(OAc)2 at 100 °C to give 43a-h, ([NBu3]+ modified silica: 43a0.5Pd, 43a1Pd, 43a3Pd; N-methyl-Imidazolium modified silica: 43b0.5Pd, 43b1Pd, 43b3Pd; NH2-modified silica: 43c0.5Pd, 43c1Pd, 43c3Pd; NHMe-modified silica:43d3Pd; NEt3-modified

silica:

NHPh-

43e3Pd;

modified

silica:43f3Pd;

NHCH2CH2NH2-modified silica: 43g3Pd; NH(CH2)2NH(CH2)2NH2-modified silica: 17



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43h3Pd).91 TEM results indicated that particle size was 5 nm. The Suzuki reaction of Ph(OH)2 with p-bromoanisole catalyzed by 43c0.5Pd using K3PO4 in toluene at 110 °

C afforded biphenyl in 90% yield, while substituted bromoarenes coupled with

Ph(OH)2 using 43c3Pd-43h3Pd to give their products in 75–98% yields. However, catalysts (43a3Pd) and (43b3Pd) showed a poor activity. Long alkylamine chains containing catalysts (43g3Pd) and (43h3Pd) gave promising results, in terms of activity and recyclability. These catalysts could be recycled 4-6 times with Pd leaching. Sarkar group prepared a SBA-16 supported Pd complex (44) by treating chloropropylated SBA-16 with PdCl2 in THF at 50 °C.92 TEM images of (44) indicated a well-ordered mesopore structure. The Suzuki reactions of aryl-iodides and bromides proceeded in coupling with arylboronic acids under optimized condition (0.01 mol% Pd catalyst, K2CO3, EtOH/H2O, 90 °C) to give their products in 88−94% yields. This catalyst provided 53−73% yields with chloroarenes. Generally, this catalyst is more active than SBA-16 supported Pd–guanidine catalyst.93,94 TEM images of reused (44) confirmed its heterogeneous pathway. Fabrication of Pd@meso-SiO2 (45) was achieved by coating a thin layer of silica on Pd/C using TEOS and CTAB followed by calcination treatment (Scheme 15).66 TEM studies of (45) showed the distribution of PdNPs (5 nm) on the surface of silica.

Scheme 15. Synthesis Route to the Composite Nanoreactor (45)

Under optimized condition (K2CO3, 10mg Pd catalyst, ethanol, 80 °C), this nanocomposite showed a 52−99.5% conversion in the coupling of aryliodides with Ph(OH)2 derivatives, while C6H5Br afforded only a 32% conversion. This catalyst could be reused four runs without deactivation. Although catalysts (45) and Pd@C composites (8)66 had the same initial 4.0 wt% Pd loading, Pd leaching from (8) was 8 times higher than that of (45), showing its performance in a stable manner. 18


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A series of SiliaCatPd0-Hydrogel catalysts, SiliaCatPd0-1 (46a) (0.03mmol/g Pd loading), SiliaCatPd0-2 (46b) (0.112 mmol/g Pd loading), SiliaCatPd0-3 (46c) (0.148 mmol/g Pd loading) and SiliaCatPd0-4 (46d) (0.163 mmol/g Pd loading)) were prepared by the hydrolysis and condensation of MeSi(OEt)3 and Si(OEt)4, and consequently, doped with K2PdCl4 followed by hydrogenation.95 SEM image of (46a) revealed the size of organosilica (60-125 µm). The catalyst (46a) promoted 95−100% yields in the Suzuki coupling of aryl- and heteroaryl- iodides and bromides with Ph(OH)2 under optimized conditions (K2CO3, 0.1−0.5%mol Pd catalyst, MeOH or EtOH). The catalyst could be reused seven cycles with minimal leaching of Pd and Si. A hot filtration test showed its heterogeneous manner. A phosphine-free Pd/FSG nanocatalyst (47) was prepared by the reaction of Pd(OAc)2 and fluorous silica in perfluorooctane.96 The size of PdNPs was 2.9 nm. When this catalyst was tested in the Suzuki reaction of Ph(OH)2 with aryl bromides using K2CO3, and 0.1 mol% Pd catalyst in MeOH/H2O, a 85−95% yield was obtained, while aryl iodides gave their products in 95−98% yield. Aryl chlorides gave only 7−21% yields. This catalyst could be reused five times with a low Pd leaching. We compile reports on some silica based Pd catalysts for Suzuki coupling reaction (Table 1).

19



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20


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Table 1. Catalytic Reactions Using Silica Supported PdNPs and Their Catalytic Performances in Suzuki Cross-Coupling Reactions catalyst

reaction conditions

Pd size

yield (%)

recycle/leaching

ref

2.9 ± 1.4

>95

NR

30

5–6

70−96

3/NR

31

10–15

58-98

6/NR

33

10

99

5/NR

97

NR

48−53

3/NR

98

(nm) Pd/SH-SiO2 (48)

Pd/NH2-SiO2 (49)

PdNP@PPh2– SiO2 (50) Pd/SBA-15 (51)

Pd/SiO2 (52)

21



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Page 22 of 60

PdNPs@SiO2 (53)

Pd/SiO2 NPs (54)

Pd-MCM-48 (55)

3.5 ± 1.0

78–92

4/NR

99

3–5

89–97

4/NR

100

4–7

85–99

NR

101

NR

99

9/NR

102

NR

88

9/NR

102

8

70–99

No recycle/NR

103

SiliaCat S-Pd (56)

SiliaCat DPP-Pd (57)

Pd(0)/SDPP (58)

22


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MCM-Py-Pd-48 (59)

Pd-CS@SiO2 (60) NR = Not reported.

23


NR

89–96

5/NR

104

>100

68–95

5/NR

105


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Recent Progress in Heck Cross-Coupling Reactions. The Heck reaction is a fundamental synthetic transformation that has been used for the synthesis of alkenes from the coupling of alkenes with aryl halides in the presence of Pd catalyst. Such phosphine-/ligand free Pd catalysts are regarded as eco-friendly methods for use in Heck reaction,39,106−108 which is ubiquitous in pharmaceutical industries. Moreover, silica materials are one of the most solid supports for PdNPs in terms of applications to Heck reactions.109 This section focuses on the development of silica-supported Pd catalysts for use in Heck reaction. Beller et al110 first demonstrated the use of PdNPs (61) in the Heck reaction using Pd colloids derived from tetraoctyltriethylboronate and [PdCl2]. This catalyst (0.05 mol% Pd catalyst) was active in the arylation of styrene or butylacrylate by activated aryl bromides but showed moderate or little activity for deactivated aryl bromides and aryl chlorides, respectively (Scheme 16).

Scheme 16. PdNPs (61) Catalyzed Heck Coupling Reactions

Shi group prepared a Pd–SBA catalyst (62) by reacting trimethoxysilanefunctionalized SBA-15 with Pd(OAc)2 in THF.111 TEM images permitted the morphology and distribution of PdNPs on the surface of SBA-15. Catalyst (62) performed well in Heck reactions of aryl halides with styrene (88−99% yield) and methylacrylate (91−99% yield) using Et3N and 0.02 mol% Pd catalyst in air at 120−170 °C. This catalyst could be reused over five times without Pd leaching. Moreover, this catalyst performed well compared to other heterogeneous Pd catalysts112 owing to high dispersion of Pd on the surface and pores of host. The PdNP/Sα−, Sβ−, and Sγ−CD catalysts (63a-c) were prepared by refluxing a 24


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mixture of silica chloride and α−, β−, and γ−CDs followed by treated with Pd(OAc)2. 113

The particle size for (63b), (63c), and (63a) was 3, 5, and 10 nm, respectively,

while the size of silica was in µm. Among them, the catalyst (63b) exhibited an optimal activity for Heck reaction of arylhalides with stilbenes to give trans products (61−99%) with good selectivity using K2CO3 and 0.05mg Pd catalyst in H2O reflux. This catalyst could be reused five times with a low extent of Pd leaching. A

semi-heterogeneous

PdNPs

stabilized

on

the

surface

of

(tris(hydroxymethyl)aminomethane)‐functionalized SiO2, SiO2-Tris-PdNPs (64) was reported (Scheme17).114 TEM image of (64) showed that PdNPs had diameters of 11 nm.

Scheme 17. Synthetic Route for Preparation of Catalyst (64)

The efficiency of (64) was studied for Heck reactions of aryl bromo-/iodo or chloro derivatives and olefins under optimized conditions (0.0002 mmol Pd catalyst, NaHCO3, DMF, 140 °C). Depending on the nature of substituents in arylhalides and olefins used, the yield of coupling product varied from 48 to 99%. In this reaction, methylacrylate was used to suppress double arylation and enabled the reaction faster than that of styrene. Catalysts were recycled seven times without any loss of activity. Hot filtration and poisoning tests showed its homogeneous and heterogeneous pathways. A Pd(0)-MCM-41 (65) nanocatalyst was prepared by incorporation of Pd(II) into silica followed by reduction.115 The channels were less ordered in (65) compared with MCM-41, indicating amorphization of MCM-41. This catalyst was active for the Heck reaction of aryl halides with styrene or n-butylacrylate using 0.05g Pd catalyst, NaOAc, and DMF at 80−100 °C. Iodobenzene gave 88 and 100% conversion with styrene and n-butylacrylate, respectively; whereas C6H5Br displayed a 56 and 76% 25



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conversions and chloroarene gave 15 and 45% conversions. Note that only a few heterogeneous PdNP catalysts are capable of activating C6H5Cl in the Heck reaction.116 This catalyst could be reused four times with high selectivity. The PdNPs on silica-bonded N-propylmorpholine (PNP-SBNPM, 66) were produced by reacting SBNPM with Pd(OAc)2 in EtOH.117 TEM images showed location of spherical PdNPs (7 nm) onto supports. A 90% yield was achieved in the reaction of aryliodide with styrene under optimized condition (2.4 mol% Pd catalyst, Na2CO3, DMF, 120 °C), but an increase in reaction time was necessary to obtain 75 and 55% yields, respectively, for the reaction of bromo- and chloroarenes. This protocol is a one-pot route for the preparation of (E)-1,4-distyrylbenzenes in 78–90% yields from the reaction of p-substituted styrenes with aryl di-halides. High yields with a minor Pd leaching were found in a six-run recycling experiment. Kalbasi et al118 prepared a composite Pd-PVP/KIT-6 (67) by heating a mixture of PVP/KIT-6 and Pd(OAc)2 in aqueous acidic solution followed by reduction with hydrazinehydrate. TEM images of (67) showed the location of PdNPs (4.5 nm) inside the channels. Under optimized condition (0.14g Pd catalyst, K2CO3, MeOH/H2O, 60 °

C), although C6H5I coupled with a high efficiency (97% yield), 12 h reaction time

was needed for deactivated iodobenzenes to give 40−98% yields. However, chloroand bromo-benzenes afforded the coupling products in 90−98% yields, but significantly longer reaction times were needed, compared to that of C6H5I. This catalyst can be reused eight runs with a lower Pd leaching. Compared to Pd-SBA-15 (27a),77 this catalyst displayed more recyclability as 3D porous networks of KIT-6 permitted a faster diffusion of reactants, and hence, avoided pore blockage, provided more adsorption sites and prevented the leaching of PVP and PdNPs into the solution. Bazgir75 investigated the activity of catalyst (25) in the Heck reaction of aryl halides with olefins under optimized conditions (K2CO3, 0.5−1.2 mol% Pd catalyst, N-methyl-2-pyrrolidone, 100 °C). A 49−91% yields were obtained in coupling of arylhalides with styrene and aliphatic alkenes. Aryl chlorides also coupled to afford 65−92% yields. Under optimized conditions (0.5mol% Pd catalyst, solvent-free, 130 °

C), catalyst Pd(0)/SDPP (58) performed the Heck reactions of iodo- and

bromo-benzenes to give the desired products in 60−99% yields.103 For coupling of deactivated aryl bromides with n-butylacrylate, n-Bu4NBr additive was applied, 26


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resulting in 52−89% yields of their products. The Heck reaction of sterically hindered 1-iodonaphthalene afforded the desired products in 78−90% yields. This catalyst could be reused six times with a low level of Pd leaching. The Pdnp-nSTDP catalyst (22) was used in the Heck reaction of aryl- iodides and bromides with styrenes to give the desired products in 90−95% yields under optimized conditions (K2CO3, 0.01 mol% Pd catalyst, DMF/H2O, 85

°

C).23

Arylchlorides gave 88−92% yields only at longer reaction time. Under MW irradiation, the desired products were formed in 87−96% yields in 8−20 min using the same reaction conditions. Finally, 75−88% yields of star-shaped products were obtained by the Heck reaction of 1,3,5-tribromobenzene with styrenes under MW irradiation. Catalyst MCM-Py-Pd-48 (59) catalyzed the Heck reaction of C6H5I with acrylates, isopropylacrylamide, and styrene in the presence of Na2CO3 in aqueous DMA with 0.13 mol% Pd catalyst at 130 °C to yield the desired products in 87−96% yields.104 The coupling of arylbromides with isopropylacrylamide and styrene afforded the desired products in 82−95% yields. This catalyst had a better activity than a hybrid silica supported Pd-complex.119 It could be reused five times with the loss of Pd. Two conditions were suitable for the Heck coupling of iodoarenes, but not for bromoarenes using the catalyst SiO2@PdNP (53).99 The coupling of C6H5I with olefins using 10 mmol−1 Pd catalyst and CTAB in water afforded the desired products in 72−90% yields. The coupling of p-bromo-o-iodoanisole yielded (E)-ethyl 3-(4-bromo-2-methoxyphenyl) acrylate product, while p-bromotoluene failed to react, showing the selectivity. Amine-functionalized SBA-15 anchored Pd(II), SBA-15/CCMet/Pd(II) (68) was prepared via immobilization of PdNPs on SBA-15 followed by treatment with PdCl2.120 TEM images of (68) displayed a pore diameter size of 7.58 nm. After optimizing the conditions (1.0 mol% Pd catalyst, Et3N, DMF), the coupling of aryl-iodides or bromides with styrenes afforded the corresponding trans-stilbenes in 80-98% yields, while arylchlorides gave 85−90% yields. This material could be reused five times without Pd leaching. This protocol resulted in higher conversions and yields compared to others. 27



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A SiO2/PdNP/porous-SiO2 nanosphere (69) was obtained with a uniform nanoporous shell using an optimal etching time of 80 min (Scheme 18).121

Scheme 18. Schematic Representation of the Synthesis of Nanocatalyst (69)

TEM images showed that 5 and 20 nm sized PdNPs were anchored to silica. It was efficient in the coupling of aryl iodides with styrene to form trans-stilbene in 100% yield using 10 mg Pd catalyst, Et3N as a base in DMSO at 120 °C. The coupling of bromo and chloro-benzenes was not reported. The advantages of using this catalyst (69) in Heck reactions are: (i) large number of PdNPs are incorporated in supports, thus conferring high activities (ii) nanoporous SiO2 around PdNPs permitted the reactants to reach the surface of PdNPs and hindered PdNPs aggregation (iii) Easily dispersed in solutions, thus recovered by simple centrifugation. Nanostructured Pd containing modified silica (70) was prepared by hydrolysis– polycondensation

of

TEOS

and

N,N-dimethyl-pyridin-4-yl-(3-triethoxysilyl-propyl)-ammoniumiodide followed by treatment with Pd(OAc)2 (Scheme 19).122 TEM images of (70) revealed the nanostructured morphology with a high degree of regularity.

Scheme 19. Synthesis of the Catalytic Material (70)

This catalyst was used in the Heck reaction of aryl-iodides and -bromides with methylacrylate and styrene to give their products in 70−90% yields under optimized 28


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conditions (Et3N, 0.1mmol Pd catalyst, refluxed CH3CN). However, aryl chlorides did not couple. It could be recycled five times with a low level of Pd leaching. A Pd-supported catalyst (71) was prepared by anchoring phosphine ligand on Ph-SBA-15 followed by treatment with Pd(II) (Scheme 20).123 TEM images of (71) indicated the presence of well-ordered hexagonal pore channels.

Scheme 20. Schematic Illustration for the Preparation of Catalyst (71)

This catalyst was employed in the Heck reaction of p-NO2-C6H4Br and methylacrylate under optimized conditions (3.0 mol% Pd catalyst, nBu4NOAc, supercritical CO2) to give the desired product in 98% yield. This catalyst could be reused several times without loss of activity. Banerjee et al101 reported on Pd-MCM-48 catalyst (55) for Heck reactions of aryl-bromides

and

iodides

with

butylacrylate,

acrylonitrile,

styrene,

1-methoxy-4-vinylbenzene using 0.6 wt% Pd catalyst and NaOAc in DMF at 100 °C to afford the desired products in 85−98% yields under ligand-free and aerobic conditions. Because of its large surface area, higher metal dispersion and interpenetrating network of 3D pore sizes, this catalyst showed excellent activity. However, chloroarenes were not active. This catalyst could be reused four cycles without significant loss of activity. Poly(N-vinylpyrrolidone)-grafted silica Pd complex (72) was active in Heck reactions using DMF, K2CO3, and 0.5 mmol% Pd catalyst at 120 °C.128 Aryliodides reacted with n-butylacrylate or styrene to produce the desired products in 93−95% yields, while bromobenzenes gave 80−95% yields. Chloroarenes were active only at high temperature or using TBAB additive to give their products in 40−90% yields. 29



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This catalyst could be recycled seven times without loss of activity. The heterogenesity of (72) was confirmed by a hot filtration test. Functionalized SiO2-Pd nanocomposites (39a) and (39b) performed well in the Heck coupling of C6H5I with styrene in the presence of 0.1 mol% Pd catalyst using 1,3,5-trimethoxybenzene at 100 °C to afford 76% and 71% conversions for (39a) and (39b), respectively.87 When the same reaction continued for a longer reaction time, an excellent yield was obtained. These catalysts could be reused eight times without loss of activity. The SiO2-acac-PdNPs (73) was active in Heck reaction of aryl-iodides and bromides with acrylates to produce the desired products in 72−93% yields using NMP, NaHCO3, and 2 × 10−4 mmol Pd catalyst at 140 °C.130 Chloroarenes gave their products in 55−80% yields. This catalyst could be recycled nine times with a low extent of Pd leaching. The nanocatalyst Pd-MCM-41 (12) catalyzed the Heck coupling of aryl iodides with styrene and n-butylacrylate under optimized conditions (solvent-free condition, 0.005g Pd catalyst, n-Pr3N, 130 °C) to give 82−95% yields.41 Arylbromides afforded 80−92% yields, but a longer reaction time was needed. Clark et al106 prepared PdNPs entrapped in silica gel (74) by modifying the silica using aminopropyl(triethoxy)silane and 2-pyridinecarbaldehyde followed by complexation with Pd(II) (Scheme 21). This catalyst exhibited a pore diameter size of 6−80 nm.

Scheme 21. Preparation of the Chemically Modified Mesoporous Silica Gel and Immobilized Pd Catalyst (74)

This catalyst was applied to the Heck reaction of methacrylate with C6H5I or p-iodophenol in the presence of 0.2 g Pd catalyst, Et3N in CH3CN to afford 82 and 31% conversion, respectively. Allyl alcohol showed 21 and 15% conversion for C6H5I and 4-iodophenol, respectively, while but-3-en-2-ol gave 31−41% conversion 30


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alongwith side products. Thus, the activity and selectivity was poor, owing to the formation of triethylaminehydroiodide, which was likely to become adsorbed on surface of the catalyst and blocked active sites. The catalyst could be reused five times without Pd leaching. Palladium doped SiO2 nanoparticles (Pd/SiO2 NPs, 54) nanocatalyst was active in the Heck reaction of iodo- and bromoarenes with olefins to give their products in 89−97% yields under optimized condition (Et3N, dodecane, 0.75wt% Pd catalyst, DMF, MW).100 Chloroarenes failed to react. SBA-16 supported 1,2-diaminocyclohexane Pd-complex (44) catalyzed the Heck reactions of aryliodides with butylacrylate, isopropylacrylamide, and styrene to give their products in 91−97% yields under optimized conditions (500mol Pd catalyst, Na2CO3, DMA/H2O, 125 °C).92 Aryl bromides promoted the coupling to give the desired products in 89−96% yields, while arylchlorides gave 48−71% yields. The catalyst was reused five cycles with a minimal Pd leaching. Polymer encapsulated silica supported PdNPs (28a and 28b) were active for Heck reactions of C6H5I with olefins to give their products in 30−95% yields using NMP, and N-methyldicyclohexylamine at 140 °C.78 In this reaction, catalyst (28a) was more active than (28b), and could be recycled without leaching. We compile reports on the use of some silica based Pd catalysts for Heck coupling reaction (Table 2).

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Table 2. Catalytic Reactions Using Silica Supported PdNPs and Their Catalytic Performances in Heck Cross-Coupling Reactions catalyst

reaction conditions

Pd size

yield

recycle/leaching

ref

(nm)

(%) 7/2.4%

34

88

7/0.8%

34

9−97

4/NR

74

88

7/0.9%

80

89

7/0.7

82

96

1/0.23

84

10−160 85

SiO2-SH•Pd (14)

1.8 ±

SBA-15SH•Pd (15)

2.5 SiO2–DEA–DABCO–

NR

Pd(0) (24)

2.0 ±

P.SBA-15SH•Pd (33)

0.5 2.0 ±

m-MCF-SH•Pd (34)

0.5

>5

SBA-SH•Pd (36a)

32


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SBA-15-NH2•Pd (36b)

5−10

97

NR/35

84

Pd/NH2-SiO2 (49)

5-6

80−95

3/NR

31

3-5

89−97

4/NR

100

Pd-CS@SiO2 (60)

>100

81−92

5/NR

105

Pd0/SiO2 (75)

95

5/Pd leaching

SiO2-pr-NH-cyanuric-S H-Pd (79) Si-OPPh2-Pd (80)

(0.1 ppm)

(low ppm)

HMS-OPPh2-Pd (81)

12

NR = Not reported.

34


90

5/Pd leaching (low ppm)

127

130

130

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Recent Progess in Sonogashira Cross-Coupling Reactions. Pd-catalyzed Csp2−Csp cross-coupling reaction developed by Sonogashira has had a significant impact in the field of pharmaceutical synthesis.131−135 It is a versatile route for the preparation of arylacetylenes by the coupling of alkynes with aryl or alkenylhalides (or triflates) with or without Cu(I) co-catalyst using PdNPs and a base in solvents. Although Cu(I) facilitates the reaction by the in situ generation of Cu acetylide, it causes the induction of so-called Glaser type oxidative homocoupling of alkyne to afford a diyne.136 In order to suppress the formation of this byproduct, several procedures were developed. Such reactions are known as copper-free Sonogashira reactions. Amine-free, ligand-free, and solvent-free conditions have been performed since Cu-free variant requires the use of amine, which can be detrimental to the environmental and economic advantages of this methodology. Aryl- iodides, bromides, and triflates are most used coupling partners, but the coupling of aryl chlorides remains as a challenging task. Owing to benefits, chemists have attempted to capture some achievements in moving to a greener pharmaceutical industry.137 Nanostructured Pd(0) xerogels (46a-46d) showed 99−100% product yields in the coupling of phenylacetylene and iodobenzenes using trimethylamine or K2CO3 as a base and 0.1 mol% SiliaCat Pd0 in EtOH.95 The sol–gel encapsulation of Pd prevented the use of deaerated conditions and avoided the formation of homocoupled acetylenes. Trademarked SiliaCatPd0-Hydrogel is now commercially available, owing to its excellent stability and versatility of use among–Pd heterogeneous catalysts.46,135 A Pd-LHMS (82) catalyst was obtained by the reaction of Pd(II) with 2D-hexagonal organosilica (Scheme 22).136 HR-TEM images of (82) showed the hexagonal arrangement of the pore channels array with pore dimensions of 2.6 nm.

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Scheme 22. Synthetic Pathway for the Formation of Pd-LHMS (82)

This catalyst showed 72−90% yields in the Sonogashira coupling of aryl-iodides and -bromides with phenylacetylene using 0.03 g Pd catalyst and hexamine as a base in the absence of co-catalyst in water or DMF at 120 °C. This catalyst could be reused four times without Pd leaching. Under optimized condition (0.05 mol% Pd catalyst, K2CO3, ethyleneglycol, 120–130°C), catalyst Pd/NH2-SiO2 (49) underwent Sonogashira reaction of iodobenzenes with phenylacetylene to give their products in 85−98% yields.31 However, C6H5Br was less efficient owing to its electronic effect. Neither copper/other ligands were used nor byproducts were observed compared to other nanocatalysts.138,139 The catalyst was reused three times with a minimal Pd leaching. Nanostructured Pd(0) xerogel (46a) was active in the coupling of aryliodide with phenylacetylene using K2CO3 and over 0.1 mol% Pd catalyst in refluxing MeOH or EtOH to afford their products in 100% yields.95,140 Under MW irradiation, iodoarenes were coupled with phenylacetylene using 0.1mol% Pd catalyst in MeOH to afford 88−100% yields, while bromobenzenes gave 4−54% yields, due to the stability of the C-Br bond. However, bromoarenes afforded the desired products in 18−100% yields using 0.5mol% Pd catalyst under MW irradiation. The catalyst could be reused five cycles with a low level of Pd leaching. Amine-modified SBA-16 supported Pd-complex (44) was active in the Sonogashira reaction of aryl- iodides and bromides to give their products in 90−96% using piperidine and 200mol Pd catalyst under copper- and solvent-free conditions at 80 °C.92 Chloroarenes afforded 68−72% product yields. This catalyst was reused without loss of activity, showing a better activity than Pd(II)-Schiff-base supported on multi-walled CNT.141 36


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Nanocatalyst SiO2-PEG-PdNP (41) was used in Sonogashira reactions of iodoaryls with alkynes to afford 78−95% yields using K2CO3, 1.0 mol% Pd catalyst, and DMF at 110 °C under amine-, Cu- and phosphine-free conditions.89 Aryl bromides did not couple. This catalyst could be recycled four times without decreasing activity. MCM-41 and SBA-15 supported-Pd catalysts (83) and (84) were explored in the MW-assisted Sonogashira reaction of phenylacetylene with C6H5I to give 13% yield using 50mg Pd catalyst, and K2CO3 under solvent-free conditions.142,143 However, the use of KF/Al2O3 enhanced the yield to 72%, while Pd-Y (85), Pd-VSB-5 (86) and Pd-SBA-15 (84) gave 9–70% yields. Among liquid bases, DBU gave 93% yield. A deprotonation mechanistic pathway was proposed for this reaction. Organopalladium(II)-functionalized SBA-16 (Pd(II)-PMO-SBA-16, 87) was obtained by co-condensation of Pd[PPh2(CH2)2Si(OCH2CH3)3]2Cl2 with TEOS.144 SEM images of (87) showed its length (2 mm) and diameter (500 nm). Under optimized conditions (CuI, n-decane, H2O, 90 °C and 0.6 mol% Pd catalyst), aryliodides coupled with alkynes to give 86–98% yields, but arylbromides afforded 85–96% yields if 0.9 mol% Pd catalyst was used. The efficiency of this catalyst was comparable to that for Pd(PPh3)2Cl2 (88), but faster than that for Pd(II)-SBA-15 (89) under identical conditions. The ordered and interconnected mesopores of (87) facilitated the transport of reactants and products without pore blockage. The catalysts (87) and (89) could be reused five times without Pd leaching, indicating inhibition of leaching by Pd(II) embedded in the silica. Farjadian and Tamami145 reported on the preparation of PdNPs supported on poly(4-vinylpyridine)-grafted silica (90) (Scheme 23). SEM and TEM images of (90) showed the dispersion of PdNPs (30–50 nm) throughout the catalyst surface. Scheme 23. Synthesis of the Poly(4-vinylpyridine)-grafted Silica-Pd(0) (90)

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This catalyst underwent the coupling of iodo- and bromo-arenes with phenylacetylene to give their products in 40–93% yields under optimized condition (NMP, K2CO3, 0.3mol% Pd catalyst, 120 °C). Iodoarenes reacted faster than bromoarenes, while aryl chlorides needed TBAB to promote the reaction and gave 65−85% yields. This catalyst could be reused six runs with neglibible Pd leaching. For the activation of aryl chlorides, earlier findings showed the requirement of high catalyst loadings, elevated reaction temperatures, and co-catalyst CuI.145,146 However, Pd@SiO2 catalyst (33) was capable of coupling o-, m- and p-chloropyridine or thiophene derivatives with phenylacetylenes under optimized conditions (0.5 mol% Pd catalyst, piperidine, TBAB, H2O) to afford the corresponding products in 82−96% yields.80 This protocol was the first report on the heterogeneous Sonogashira reaction of heteroaryl chlorides in water. A pH-responsive silica-supported PdNPs (Pd-5C-1N, 91), was prepared and explored its application for the Sonogashira reaction.22 TEM image of (91) showed the location of PdNPs (size = 0.4 nm) in the pores of microspheres (200–300 nm). Under optimized conditions (Et3N, H2O-ether, and 1.0 mol% Pd catalyst), 73−99% and 81−86% yields, respectively, were achieved in the coupling of aryl- and heteroaryl-iodides with alkynes, while arylbromides gave 33−46% yields. The recycling ability of (91) was examined by changing the pH of the reaction mixture at different ranges. Aryl chlorides gave 81−91% yields in comparison with other heterogeneous catalysts for copper-free reactions.147 The use of K2CO3 and refluxing water had no effect on the grafted starch during the reaction. This catalyst could be reused five cycles with negligible Pd leaching. MCM-48 supported 2-pyridinylmethanimine (Py) Pd-catalyst (59) promoted the coupling of aryl- iodides and bromides with phenylacetylene under optimized condition (Piperidine, 0.013mol% Pd catalyst (for aryliodides) 0.025 mol% Pd catalyst (for arylbromide), solvent-free, 80 °C) to give their products in 93−96% and 91−94% yields, respectively.104 It could be reused five cycles with a low levels of Pd leaching. This catalyst showed a better performance than di-(2-pyridyl)-methylamine hybrid silica-PdCl2.119 Easy synthetic procedures, long shelf-life, stability, and compatibility make this catalyst an ideal system.

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Mercaptopropylated SBA-15 supported PdNPs (13) was efficient in the Sonogashira reaction of iodo- and bromobenzenes with alkynes to give desired products in 91−98% and 88−96% yields, respectively, using 0.1 mol% Pd catalyst, and piperidine under solvent-free condition.70 This catalyst could be reused six times without loss of activity. This catalyst promoted the reaction, which was twelve times faster than Pd(II)-Schiff-base complex supported on multi-walled CNT under copper-free condition.141 Under optimized condition (CH3CN, Et3N, 0.1mmol Pd catalyst), silica PdNPs (92) catalyzed the Sonogashira coupling of heterocyclic- and steric-substituted aryliodides with alkynes to give their products in 65–82% yields.131 Although the immobilized pyridine located on the surface of silica in (92) caused the low Pd leaching, the anchoring of Pd species minimized the deterioration, thus allowing this catalyst to be recycled. We compile reports on the use of some silica based Pd catalysts for Sonogashira coupling reaction (Table 3).

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Table 3. Catalytic Reactions Using Silica Supported PdNPs and Their Catalytic Performances in Sonogashira Cross-coupling Reactions catalyst

reaction conditions

Pd size

yield

(nm)

(%)

3−5

88−96

PdNPs–SSS (76)

8

81−91

Pd(0)-MCM-41 (93)

Functionalized Silica Matrices and Palladium: A Versatile

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